On January 31 an undertaking was submitted for consideration, which proposed a number of obligations by Clarence Coal to deliver benefits to workers, industry and the community.

The proposed undertaking was submitted after an investigation into an incident at the mine on July 4 2018 in which two workers were seriously injured when material from a rib and cornice fell on them.

"The regulator considered the undertaking, and on February 24 2020 rejected the proposal, being of the view the circumstances of the incident warranted strong specific and general deterrence," it said.

A Centennial spokeswoman told Australia's Mining Monthly: "Centennial will continue to cooperate with the regulator in order to further develop a WHS undertaking that is an accepted enforceable undertaking".

The regulator had earlier warned that a risk-based approach for rib hazard identification and assessment was crucial and rib conditions had be assessed in three phases during the life of mine workings after the rib fall incident at Clarence.

Falling coal rib struck two workers - who are father and son - in the underground workings of the mine when the strata failed on July 4 2018. One of the workers suffered serious injuries.

The regulator said rib conditions should be assessed as early as possible during the cutting of the development faces.

It said that assessment should continue through the formation of the pillars and while the pillars stood because there was often time-dependent movement, and during and after secondary extraction as stress abutments developed in pillar extraction.

"Competent geotechnical personnel should carry out assessment of rib stability should be carried out, which should consist of, at least, detailed rib mapping throughout the life of the workings and installation of pillar rib instrumentation such as extensometers and instrumented rib bolts," it said.